The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to fluids used in treating a subterranean formation. In particular, the invention is related to aqueous wellbore treatment fluids containing a heteropolysaccharide, methods of forming such fluids, and uses thereof, and methods of breaking such fluids.
Various types of fluids are used in operations related to the development and completion of wells that penetrate subterranean formations, and to the production of gaseous and liquid hydrocarbons from natural reservoirs into such wells. These operations include perforating subterranean formations, fracturing subterranean formations, modifying the permeability of subterranean formations, or controlling the production of sand or water from subterranean formations. The fluids employed in these oilfield operations are known as drilling fluids, completion fluids, work-over fluids, packer fluids, fracturing fluids, stimulation fluids, conformance or permeability control fluids, consolidation fluids, and the like.
Fluid technologies incorporating a gaseous component or a supercritical fluid to form a foam or energized fluid are commonly used in the stimulation of oil and gas wells. For example, some viscoelastic fluids used as fracturing fluids contain a gas such as air, nitrogen or carbon dioxide to provide an energized fluid or foam. Such fluids are commonly formed by injecting an aqueous solution (“base fluid”) concomitantly with a gas, most commonly nitrogen, carbon dioxide or their mixtures, into the formation. Among other benefits, the dispersion of the gas into the base fluid in the form of bubbles or droplets increases the viscosity of such fluid and impacts positively its performance, particularly its ability to effectively induce hydraulic fracturing of the formation, and also its capacity to carry solids (“proppants”) that are placed within the fractures to create pathways through which oil or gas can be further produced. The presence of the gas also enhances the flowback of the base fluid from the interstices of the formation and of the proppant pack into the wellbore, due to the expansion of such gas once the pressure is reduced at the wellhead at the end of the fracturing operation. Other common uses of foams or energized fluids include wellbore cleanout, gravel packing, acid diversion, fluid loss control, and the like.
The viscosity of the fluid in which the gas is dispersed affects the resulting viscosity and stability of the foam. In general, foams are more stable and viscous as the viscosity of the base fluid increases. For this reason, high molecular weight polymers are commonly added to increase the viscosity of the base fluid. Commonly used polymers for fracturing applications are polysaccharides such as cellulose, derivatized cellulose, guar gum, derivatized guar gum, xanthan gum, or synthetic polymers such as polyacrylamides and polyacrylamide copolymers.
Incorporating crosslinkers into the fluid further augments the viscosity of the base fluid. Crosslinking consists of the attachment of two polymeric chains through the chemical association of such chains to a common element or chemical group, whereas such element or group is referred to as the crosslinker. Typical crosslinkers are polyvalent metal ions, more often zirconium or titanium ions, or borate ions. Crosslinking may be sensitive to the prevailing pH. For example, crosslinking with borate ions can be performed only in alkaline media (pH>8). pH-regulating systems (“buffers”) are often required to achieve effective crosslinking with metal ions.
Foamed and energized fracturing fluids invariably contain “foamers”, most commonly surfactant or blends of surfactants that facilitate the dispersion of the gas into the base fluid in the form of small bubbles or droplets, and confer stability to the dispersion by retarding the coalescence or recombination of such bubbles or droplets. Foamed and energized fracturing fluids are generally described by their foam quality, i.e. the ratio of gas volume to the foam volume. If the foam quality is between 52% and 95%, the fluid is conventionally called foam, and below 52%, an energized fluid. However, as used herein the term “energized fluid” is defined as any stable mixture of gas and liquid, notwithstanding the foam quality value.
Hydraulic fracturing is a stimulation technique routinely performed on oil and gas wells to increase fluid production from subterranean reservoirs. Specially engineered fluids, including energized fluids thickened with viscoelastic surfactants or polymeric gelling agents, are often pumped at high pressures and rates into the reservoir to be treated, causing a fracture to open. Proppants, such as ceramic beads or grains of sand, are slurried with the treating fluid (also referred to as carrier fluid) to keep the fracture open once the treatment is completed. Hydraulic fracturing creates high-conductivity communication with a large area of a formation and bypasses any damage that may exist in the near-wellbore area. It is therefore important for the treatment fluid to have enough viscosity to suspend and carry the proppant into the fracture zone. In some cases, however, depending upon specific subterranean formation conditions or job designs, energized fluids may not have high enough viscosity to achieve optimum proppant transportation and suspension, thereby resulting in poor proppant placement. Increased levels of viscosifying agent or surfactants may be required to achieve adequate proppant placement, thus leading to increased resource and material requirements.
The ability to formulate stable energized fluids with rheological properties suitable for fracturing operations becomes increasingly difficult as the temperature of the subterranean formation increases. The matter is worsened when carbon dioxide is present in the gas phase, since carbon dioxide exhibits high solubility in aqueous solutions. The high solubility of carbon dioxide facilitates mass transfer between bubbles and accelerates foam-destabilizing mechanisms such as so-called Ostwald ripening, which ultimately lead to phase separation and to the loss of fluid viscosity. Furthermore, carbon dioxide reacts with water to form carbonic acid. The formation of carbonic acid imposes a low pH environment for the fluid (typically in the range 3.5-4), thus impeding the necessary control of pH for efficient crosslinking with borate ions and often with other metallic ions. Exposure to low pH and high temperatures promotes degradation of the polymeric chains, particularly if polysaccharides are used as viscosifying agents, thus contributing to the referred loss of foam stability and viscosity.
Fluids containing polymers, such as those described above, are typically broken after the treatment is carried out so that the polymer, now broken, can be readily removed. Breaking agents are therefore typically added to the fluid for such cleanup, which may be delayed breaking agents, or the breaking agent may be added as a separate stage. In the case of polysaccharides, difficulty in breaking the polymer with breakers such as persulfates has been encountered, particularly at lower temperatures.
Accordingly, a need exists for improved polymer fluids that are useful for various treatments, such as hydraulic fracturing, and that can be readily removed over a wide variety of temperatures.